The wonders of the heavens

The planets have been a subject of wonder to man from earliest recorded times. Their very name, the Wandering Ones, recalls the fact that their apparent positions in the sky change continually, in contrast to the fixed stars. Greek astronomers, Ptolemy particularly, had shown how the motions of the planets, the Sun and the Moon could be accounted for if they were all supposed to move around a stationary Earth, and in mediaeval times an elaborate cosmology was created, at its most allegorical, evocative and poetic in the Paradiso of Dante. The men of the Renaissance overthrew these ideas but provided fresh cause for wonder in their place. Placed in motion around the Sun by Copernicus, their paths observed with care by Kepler, the planets led Newton to his ideas of universal gravitation. Galileo, his telescope to his eye, showed that they had discs of definite size and that Jupiter had moons, the Medicean satellites, which formed a system like the planets themselves.

The discoveries of the seventeenth century settled notions of the planets for three centuries, but within that framework a most extra-ordinary flowering of the intellect attended the working out of the ideas of Newton. Closer and closer observation showed ever more intricate departures of the paths of the planets from the simple ellipses of Kepler, and each was accounted for by ever subtler applications of mechanics as the consequence of the gravitational pull of each planet upon its fellows.